We dove the Blue Hole in May. We're not uber-experienced. We notched our 100th dive on the trip. The Hole wasn't challenging in terms of currents, but I did take note of how quickly we hit the 130 ft. limit of our dive (we'd never previously been below 100 ft.). I could see how a diver could very quickly get far deeper than planned.
Saw maybe six or seven reef sharks circling around the hole. Visibility was bad. Very windy, so sediment was getting picked up along the shelf and dumped down the hole with us.
Other fun sights on the trip included dolphins (on two separate dives) and a blue marlin, of all things, that swam right up to us during a safety stop at The Elbow.
Overall, maybe our best dive trip ever. I HIGHLY recommend Belize diving.